The Great Plains low-level jet during PECAN: observed and simulated characteristics


During the 2015 Plains Elevated Convection At Night (PECAN) field campaign, several nocturnal low-level jets (NLLJs) were observed with integrated boundary layer profiling systems at multiple sites. This paper gives an overview of selected PECAN NLLJ cases and presents a comparison of high-resolution observations with numerical simulations using the Weather Research and Forecasting (WRF) model. Analyses suggest that simulated NLLJs typically form earlier than the observed NLLJs. They are stronger than the observed counterparts early in the event, but weaker than the observed NLLJs later in the night. However, sudden variations in the boundary-layer winds, height of the NLLJ maximum and core region, and potential temperature fields are well captured by the WRF model.

Simulated three-dimensional fields are used for a more focused analysis of one PECAN NLLJ case (IOP12). While previous studies often related changes in the thermal structure of the nocturnal boundary layer and sudden mixing events to local features, we hypothesize that the spatial evolution of the NLLJ plays an important role in such events. The NLLJ is shown to have heterogeneous depth, wind speed, and wind direction. The heterogeneous NLLJ moves from west to east across the slope of the Great Plains overnight. As the NLLJ evolves, westerly advection becomes significant. This spatial and temporal heterogeneity of the NLLJ is suggested as a source of the often observed and simulated updrafts during PECAN NLLJ cases and as a possible mechanism for nocturnal convection initiation. The spatial and temporal characteristics of the NLLJ are inter-connected and should not be treated independently.

Monthly Weather Review